1. Field of the Invention
Embodiments of the present invention generally relate to nuclear medicine, and systems for obtaining images of a patient's body organs of interest. In particular, the present invention relates to a novel method and system for utilizing an adaptive framing protocol in medical imaging.
2. Description of the Related Art
Heart disease is very common. The heart can be evaluated for large vessel and small vessel disease. One by-product of small vessel heart disease is poor heart oxygenation.
Nuclear medicine is a unique medical specialty wherein radiation is used to acquire images that show the function and anatomy of organs, bones and/or tissues of the body. Radiopharmaceuticals are introduced into the body, either by injection or ingestion, and are attracted to specific organs, bones and/or tissues of interest. For example, the radiopharmaceutical (e.g., rubidium) is injected into the bloodstream.
The radiopharmaceutical produces gamma photon emissions that emanate from the body. One or more detectors are used to detect the emitted gamma photons and the information collected from the detector(s) is processed to calculate the position of origin of the emitted photon from the source (i.e., the body organ or tissue under study). The accumulation of a large number of emitted gamma positions allows an image of the organ or tissue under study to be displayed.
How fast the radiopharmaceutical is taken in by the heart indicates how quickly the heart is being oxygenated and also indicates how healthy the small micro-vessels are in the heart. The rate of absorption of the radiopharmaceutical is determined by comparing the amount of radiopharmaceutical at one time with the amount at another time.
To calculate the rate of absorption, measurements are taken at various times. Data is acquired for each patient under “rest” and “stress” conditions. Stress is usually induced through either some form of exertion (e.g., walking or running on a treadmill) or by injection of a chemical which increases the heart rate. The ratio between stress and rest in a healthy heart is about a factor of 4 and in a diseased heart the stress/rest ratio is about a factor of 1.2.
In PET studies of cardiac function, emission data are typically collected in list mode. The list is then divided into a predetermined temporal sequence of frames (using a framing protocol), an image is reconstructed from the data in each frame, and the sequence of reconstructed images analyzed for evidence of disease.
To date, framing protocols have universally been fixed for every patient. Clinicians choose some invariant sequence of framing times, which never changes. These fixed framing protocols are the same within each clinic.
For example, Lorte, Quantification of Myocardial Blood Flow with 82Rb Dynamic PET Imaging, Eur. J. Nucl. Med. Mol. Imaging (2007) 34: 1765-1774, (“Lortie et al.”) analyzes all patient data using a framing protocol that consists of 17 frames organized as 12*10 s+2*30 s+1×60 s+1×120 s+1×240 s; and El Fakhri, Absolute Quantitation of Regional Myocardial Blood Flow (MFB) Using RB-82 PET: Experimental Validation Using Microspheres, J. Nucl. Med. 2007, 48 (Supplement 2) 54P, (“El Fakhri et al.”) analyzes all patient data using a framing protocol that consists of 34 frames organized as 24*5 s+6*10 s+4*20 s. In some studies, the first frame is started on a signal derived from the data, but in all studies the timing of the frames does not depend on any features of the data.
After image reconstruction, the amount of radioactivity in the heart can be measured.
One way to estimate dynamic physiological parameters from quantitative reconstructed images is given by Lortie et al., using a one-compartment model:Cm(t)=K1e−k2t*Ca(t)  Equation (1)
where Ca(t) and Cm(t) are the measured concentrations of the radiotracer in the arterial blood and the tissue of interest, respectively. K1 is a measure of how quickly the radiotracer flows into the tissue of interest and k2 represents how quickly it flows out. To estimate the model parameters K1 and k2, least squared error minimization can be used, with each frame assigned a weight proportional to its duration in time.
The prior art analyzes small vessel disease using a fixed framing protocol which often leads to an excessive number of frames used in the analysis.
Therefore, there exists a need in the art for a protocol which is adapted for each individual patient to minimize the number of frames used in the analysis of the medical images.